Cardiac arrhythmias are responsible for more than 300,000 deaths per year in the US alone. Despite extensive research over many decades the underlying mechanisms for these arrhythmias are not completely understood. A common mechanism, believed to underlie a wide variety of arrhythmias, is the presence of ectopic focal excitations in the heart. These ectopic foci disrupt the normal sinus rhythm, and can produce triggered excitations which can lead to reentry and/or wave fractionation in the heart. Remarkably, it is not understood what determines the timing, location, and morphology of these focal excitations. Many experimental studies have shown that abnormal calcium cycling, at the single cell level, plays an essential role in the formation of these focal excitations. These studies are corroborated by gene based studies showing that specific mutations of Ca cycling proteins are found in hearts prone to ectopic activity and fibrillation. However, the detailed mechanisms linking subcellular Ca and focal excitations at the tissue and whole heart level is not known. In this project we propose to develop a multi-scale computational framework that can be used to describe the properties of Ca mediated ectopic foci. Our aim is to explore how abnormal Ca cycling at the subcellular level can summate over thousands of cells to form ectopic foci in tissue. Our computer models will shed light on the underlying mechanisms by bridging the gap between ion channels, cell electrophysiology, and tissue scale electrical activity.